This invention relates to anaerobic reactors and more particularly to the staged reactor in which the mesophyllic and thermophilic biota in both acid forming and gas forming phases are segregated into separate sections, and each section is divided into several stages in which rotating biological contactors are employed with carbon dioxide by-product recirculation from the acid forming phase of both mesophyllic and thermophilic sections for the purpose of optimizing reaction kinetics and the quality and quantity of methane gas production.
The use of anaerobic digestion for municipal and industrial waste product reduction and stabilization of the removed waste products has been widely practiced in the past, primarily because of the degree of sludge reduction and the potential for production of usable methane gas. Although these reactors were well accepted by both industrial and municipal waste disposal authorities, they are not being utilized in nearly as many applications for which they are suited primarily because of certain practical and financial considerations regarding their construction and use.
The standard practice at present is to employ a large cylindrical tank in which all anaerobic reactions occur. The gas output from the anaerobic reactor is typically a mixture of carbon dioxide and methane gas of relatively low commercial value. The produced gas is used primarily to mix the contents of the reactor by injection in lances or in a bubble gun, and the excess gas is burned in a gas burner which maintains the temperature of the liquid in the tank to the required temperature for thermophilic, more commonly, mesophyllic reaction, whichever is used in that reactor.
In industrial waste disposal plants employing waste of a biological oxygen demand much higher than that seen in municipal plants, typically higher than 1,000 parts per million BOD, the burden on the municipal plant is extreme. In these applications, the potential gas production and sludge reduction is much greater in anaerobic reactors than in aerobic reactors, so the use of anaerobic reactors in industrial applications becomes potentially extremely attractive. However, the potential has never been achieved in any available apparatus and therefore the usual practice is for the industry to simply pay heavy user fees to municipal plants where the high strength waste is mixed with ordinary municipal waste and treated in the usual municipal waste water treatment plant. This represents a waste of potentially valuable resources and also constitutes an unnecessary expense to the industry and burden on the municipal plant.
Anaerobic digesters are not sufficiently utilized by industry because the initial investment for design and installation for such a system is high, the land area required for the typical large anaerobic digestion tank is large, the energy output is insufficient and of low quality and the sludge production is excessive and of insufficient purity for safe disposal. The result of these factors is that the installation and maintenance costs extend the payback period so far into the future that the purchase of such a system becomes economically unattractive.
Thus, the industry has long been in need of an anaerobic digester that is designed to achieve high energy yield and low volumes of high purity sludge, and does so in an apparatus that is small in land use area and both mechanically and biologically reliable.